I have built two six-digit Nixie clocks with Z5660M tubes, which I bought as "new old stock" - likely unused - for about 50€ per piece over 10 years ago. Both

I have built two six-digit Nixie clocks with Z5660M tubes, which I bought as "new old stock" - likely unused - for about 50€ per piece over 10 years ago. Both of these clocks have been running non-stop since then, and they are still going strong after more than 86.000 hours of operation, with negligible loss of brightness. They are still readable in anything but direct, full-brightness sunlight, and illuminate the room nicely by night. I guess the flash memory of the Atmel microcontroller running the clock logic will lose its contents before the tubes eventually wear out.